In recent large-capacity optical communications systems, an optical signal modulated at a high bit rate of Gbit/s or more is transmitted. As the transmission distance is extended, the signal with wavelength chirping is more significantly influenced by the fiber dispersion effects, resulting in distortion of the signal pulse shape. It is, therefore, necessary to use an optical signal which is smaller in wavelength chirping. Under these circumstances, an optical signal is generated not by direct modulation of a laser diode (LD) having an extremely large chirping, but by external modulation combining a continuous-wave operated laser diode (LD).
A typical external modulator which is used in a long distance optical transmission is an LN modulator configured with LiNbO3 (LN) optoelectronic waveguide, in which an optical waveguide is coupled to an electrical waveguide. The operation principle of the LN modulator is based on the change in optical refractive index by electric field in a waveguide due to electro-optical effects, and resulting phase shift of optical signal. The above-described LN modulator is able to function as an optical phase modulator, a light intensity modulator in which a Mach Zehnder (MZ) interferometer is incorporated, or an intelligent optical switch constituted by combining many waveguides.
However, LN modulators still have many problems. Since LiNbO3 is a dielectric material, a sophisticated manufacturing technology is required in stabilizing the surface of the material and processing a waveguide. It is also necessary to use a special photolithography because of relatively long in waveguide length which is different from that for semiconductor fabrication process. Further, the size of package in which an LN modulator is loaded must be made larger in dimension. Due to these reasons, the LN modulator module is higher in manufacturing cost and an optical transmitter using it is relatively large in dimension, which are problems related to a conventional modulator.
Also known are semiconductor optical modulators, the operation principle of which is similar to that of an LN modulator. For example, these include a GaAs optical modulator in which a Schottky electrode is placed on a semi-insulating GaAs to configure an optoelectronic waveguide. Another one is an InP/InGaAsP optical modulator. In this type of modulator, driving voltage is effectively applied to a core portion of a waveguide through utilization of a hetero pn junction, in addition to good optical confinement.
However, of these semiconductor optical modulators, the former (LN modulator) has a disadvantage in that electrical loss is large due to a longer waveguide length, whereas the latter (conventional semiconductor-based modulator) is disadvantageous in that a greater light absorption due to a p clad layer results in a difficulty of designing a longer waveguide for realizing low driving voltage. In recent years, as a structure to overcome these disadvantages, proposed is a modulator in which clad layers on both sides of an InP/InGaAsP optical modulator are an n-type (a so-called nin-type structure) (for example, refer to Patent Documents 1 and 2).
FIG. 9 is a view illustrating a band diagram of the semiconductor optoelectronic waveguide which constitutes a conventional InP/InGaAsP optical modulator. The symbol 101 given in the view denotes a core layer of a waveguide; 102-1 and 102-2, first clad layers; 103-1 and 103-2, a p-type and an n-type second clad layers respectively. Further, 100-1 and 100-2 denote respectively electrons and positive holes (holes). Voltage is applied to the p-type second clad layer 103-1 and the n-type second clad layer 103-2 to induce a desired electro-optical effect on the core layer 101, thereby realizing an optical modulation. In the above-described conventional waveguides, voltage is applied to the core layer 101 by a pn junction, achieving a decreased leak current. Further, carriers generated by light absorption are allowed to flow easily to electrodes, thereby realizing in a stable operation.
However, a GaAs optical modulator equipped with a Schottky electrode has a problem that an operating voltage is elevated. Further, an InP/InGaAsP optical modulator has a problem that an operating bandwidth is narrow due to transmission loss of electrical signals resulting from a higher resistance on a p-type clad layer. In addition, a greater light absorption a p-type clad layer (described above) makes it difficult to prolong the waveguide length, thereby further reducing the operating voltage of the modulator is limited. The transmission loss of electrical signals in the InP/InGaAsP optical modulator appears in the course of charge and discharge by the pn junction through the resistance of signal lines and that of the p-type second clad layer 103-1. In particular, since the resistance of the p-type second clad layer 103-1 originates from physical properties of a material that hole mobility is low it is an unavoidable problem. In view of the above-described problems, in recent years, a nin-type waveguide structure has been proposed.
FIG. 10 is a view showing a band diagram of the nin-type semiconductor optoelectronic waveguide structure. Namely, the p and n clad layers (103-1 and 103-2) on both sides of the InP/InGaAsP optical waveguide given in a FIG. 9 are changed to those of n-type, and the modulator operation is done by applying voltage between these two n-type electrode layers. The symbol 111 given in the figure denotes a core layer of the waveguide, and 112-1 and 112-2 denote the first clad layers. This is different from the constitution given in FIG. 9 in that both of the electrode layers (114-1 and 114-2) are of n-type, and the p-type second clad layer 103-1 given in FIG. 9 is replaced by a Fe-doped semi-insulating layer 115 having a deep levels 116 and an n-type electrode layer 114-1 (for example, refer to Patent Document 1). It is noted that the n-type electrode layer 114-2 is corresponding to the n-type second clad layer 103-2 given in FIG. 9, and 110-1 and 110-2 denote respectively electrons and positive holes (holes).
In the above-described constitution, a deep Fe level 116 in the semi-insulating layer 115 acts as an ionized acceptor and, therefore, the electrical charge will bend a band to form a potential barrier to electrons. As indicated by the arrow in the figure, electrons 114-1 and holes 110-2 in the vicinity of the curved portion of the band are recombined via the deep Fe level 116 in the semi-insulating layer 115. Therefore, the potential barrier can keep its profile when excess hole are induced, and suppresses leakage current by electron flow from the layer 114-1, making it possible to apply an electric field to the core layer 111.
However, in the above waveguide structure, the density of ionized Fe deep levels changes depending on a bias because the density of the deep levels 116 is not sufficiently high. Dependence of such ionization on the bias will cause a change in thickness of a depletion layer, resulting in a failure in keeping a proportional relationship between the applied voltage and the electric field involved in the core layer 111. There is also a problem that response to a high speed modulation signals is difficult due to a relatively long interval of capture and emission of carriers by the deep Fe level 116. In another words, the modulation strength has frequency dispersion.
Further, a basic concept that “voltage is applied between two n-type electrode layers to operate a device” has been known in the field of electron devices as a so-called bulk barrier diode. An example, in which such a concept is applied to an optical modulator, is reported in “the modulator which incorporates a quantum well core layer for inducing carrier band filling effects” (for example, refer to Patent Document 2). Since this optical modulator utilizes flow of electrons traveling into and out of a quantum well, it is theoretically impossible to faster operation speed, as compared with an optical modulator using electro-optical effects.
FIG. 11 is a diagram illustrating a conventional nin-type semiconductor optical modulator. The symbol 121 given in the figure denotes an n-type third semiconductor clad layer; 122, a p-type fifth semiconductor clad layer; 123, a first semiconductor clad layer; 124, a semiconductor core layer having electro-optical effects; 125, a second semiconductor clad layer; 126, an n-type fourth semiconductor clad layer; 127 and 128, n-type electrodes; 129, a grooved electrical isolation region formed by etching. There is another report about an electrically separated structure in which a semi-insulating semiconductor is grown again on the grooved etched portion (for example, refer to Patent Document 1), which is, however, not necessarily an optimal technique for providing an optical modulator because of the more complicated structure.
The p-type fifth semiconductor clad layer 122 and the first semiconductor clad layer 123 are sequentially laminated on the n-type third semiconductor clad layer 121, and the semiconductor core layer 124 having electro-optical effects is provided so as to be held between the first semiconductor clad layer 123 and the second semiconductor clad layer 125. Further, on the second semiconductor clad layer 125 is laminated the n-type fourth semiconductor clad layer 126 having the grooved electrical isolation region 129 formed by etching. On the fourth semiconductor clad layer 126 is provided the electrode 128, and on both sides of the raised portion of the third semiconductor clad layer 121 is provided the electrode 127.
In the waveguide structure given in FIG. 11, since the n-type InP clad layer 126 is partially etched in a grooved form to provide the electrical isolation region 129, an optical transmission mode is changed at a portion where a clad layer is varied in thickness, resulting in optical scattering loss. Further, in a conventional waveguide structure, the fourth semiconductor clad layer 126 is etched in a relatively deep manner, posing a problem in controlling the etching.
In a typical structure of the nin-type InP/InGaAsP optical modulator (above explained), a waveguide portion where modulation is conducted and a connecting waveguide portion outside thereof are separated electrically by partially removing a part of the upper layer of the n-type clad layer 126, thereby resulting in formation of a recess 129 on the waveguide. This recess poses a problem that optical loss increases in association with a change in the optical transmission mode at portions from a connecting waveguide to an electrical isolation region and at portions from the electrical isolation region to a main waveguide. Further, since it is necessary to leave a high resistant clad layer having a certain thickness in the electrical isolation region (recess region), the high resistant clad layer must not be reduced in thickness, thereby making it impossible to effectively apply an electric field to the semiconductor core layer 124, which is a problem.
The present invention has been made in view of the above problems. An object of the invention is to provide a semiconductor optoelectronic waveguide having a nin-type heterostructure which enables the stable operation of an optical modulator.
Another object of the present invention is to provide a semiconductor optoelectronic waveguide which less influences transmission of an optical mode than a conventional recess-forming electrical isolation region to solve a problem of optical loss, and has a structure of electrical isolation region which is well controllable and stable.
Still another object of the present invention is to solve the above-described problem that a core layer undergoes a change in voltage in a semiconductor optoelectronic waveguide such as a nin-type InP/InGaAsP optical modulator, and realize a stable operation of the semiconductor optoelectronic waveguide.
Patent Document 1: Japanese Patent Application No. 2003-177368
Patent Document 2: U.S. Pat. No. 5,647,029